1. Field of the Invention
This invention relates generally to centrifuge rotors made from composite materials, and relates more particularly to a process of fabricating structures, including centrifuge rotors, by resin transfer molding, and the resulting structures or rotors.
2. Description of the Relevant Art
Centrifuges are commonly used in medical and biological research for separating and purifying materials of differing densities. A centrifuge includes a rotor typically capable of spinning at tens of thousands of revolutions per minute.
A preparative centrifuge rotor has some means for accepting tubes or bottles containing the samples to be centrifuged. Preparative rotors are commonly classified according to the orientation of the sample tubes or bottles. Vertical tube rotors carry the sample tubes or bottles in a vertical orientation, parallel to the vertical rotor axis. Fixed-angle rotors carry the sample tubes or bottles at an angle inclined with respect to the rotor axis, with the bottoms of the sample tubes being inclined away from the rotor axis so that centrifugal force during centrifugation forces the sample toward the bottom of the sample tube or bottle. Swinging bucket rotors have pivoting tube carriers that are upright when the rotor is stopped and that pivot the bottoms of the tubes outward under centrifugal force.
Many centrifuge rotors are fabricated from metal. Since weight is a concern, titanium and aluminum are commonly used materials for metal centrifuge rotors.
Fiber-reinforced, composite structures have also been used for centrifuge rotors. Composite centrifuge rotors are typically made from laminated layers of carbon fibers embedded in an epoxy resin matrix. The fibers are arranged in multiple layers extending in varying directions at right angles to the rotor axis. During fabrication of such a rotor, the carbon fibers and resin matrix are cured under high pressure and temperature to produce a very strong but lightweight rotor. U.S. Pat. Nos. 4,781,669 and 4,790,808 are examples of this type of construction.
Composite centrifuge rotors are stronger and lighter than equivalent metal rotors, being perhaps 60% lighter than titanium and 40% lighter than aluminum rotors of equivalent size. The lighter weight of a composite rotor translates into a much smaller mass moment of inertia than that of a comparable metal rotor. The smaller moment of inertia of a composite rotor reduces acceleration and deceleration times of a centrifugation process, thereby resulting in quicker centrifugation runs. In addition, a composite rotor reduces the loads on the centrifugal drive unit as compared to an equivalent metal rotor, so that the motor driving the centrifuge will last longer. Composite rotors also have the advantage of lower kinetic energy than metal rotors due to the smaller mass moment of inertia for the same rotational speed, which reduces centrifuge damage in case of rotor failure. The materials used in composite rotors are resistant to corrosion against many solvents used in centrifugation.
A disadvantage of composite centrifuge rotors is that the loading of the rotor due to centrifugal forces can cause delaminations and failure of the structure. Reinforcing structures such as outer shells may be necessary to provide adequate structural strength, such as disclosed in U.S. Pat. Nos. 5,362,301 and 4,790,808. Another disadvantage is that extensive and costly machining of the laminated core is required in order to form the outer shape of the rotor and to form the cell holes that receive the sample tubes or bottles containing the samples to be centrifuged.
In accordance with the illustrated preferred embodiment, the present invention is a method for fabricating fiber-reinforced composite structures, including centrifuge rotors, by resin transfer molding (RTM), and the resulting composite structures. The method basically involves loading reinforcing fibers into a mold and then injecting resin into the mold to coat the fibers to form the composite structure. Either one or two molds can be used for the process. If two molds are used, most or all the reinforcing fibers are loaded into a first mold and then cured into a porous fiber structure and then the porous fiber structure is transferred to a second mold for resin injection and curing.
One aspect of the present invention is a method for fabricating a fiber-reinforced composite structure in a single mold, including the steps of: (a) forming fabric preforms corresponding to the surface of the structure; (b) placing some of the fabric preforms into a mold; (c) placing chopped fibers in the mold; (d) placing the remainder of the fabric preforms into the mold so that the fabric preforms are adjacent to the interior surfaces of the mold and the chopped fibers are inside the fabric preforms; (e) injecting resin into the mold to coat all the fabric preforms and chopped fibers; (f) curing the resin in the mold; and (g) then removing the completed structure from the mold.
Another aspect of the method is to use two molds, a first mold to form the preforms and chopped fibers into a porous fiber structure called a xe2x80x9cbirds nest,xe2x80x9d and a second mold for the resin injection. The molds may incorporate a mold insert or mandrel that is moved from the birds nest mold to the injection mold with the birds nest structure.
Another aspect of the method is the forming of the fabric preforms, which is done by applying resin in the form of a solid powder or other forms to a piece of fabric, heating the fabric and then inserting it into a forming tool, which forms the fabric into the desired shape. The resin adds stiffness so that the fabric preform can retain its shape until it is placed into the mold for forming the birds nest and/or resin injection.
Yet another aspect of the present invention is a fiber-reinforced composite structure comprising a skin layer of reinforcing fabric adjacent the surfaces of the structure, chopped fibers distributed throughout the interior of the structure and under the skin layer, and epoxy resin throughout the structure, which binds the fabric and chopped fibers together into a fiber-reinforced composite structure. Fabric preforms are positioned adjacent to the surfaces of the structure, and provide fiber reinforcement for the entire surface. The chopped fibers are located randomly within the interior of the structure, and provide fiber reinforcement throughout the structure.
A preferred structure resulting from the resin transfer molding method is a centrifuge rotor, and so the invention encompasses both the centrifuge rotor itself and the method of making it.
The present invention provides a simple and cost-effective method of fabricating fiber-reinforced composite centrifuge rotors. The present invention uses composite materials and thus retains the advantages of all-composite construction in terms of light weight, low energy, and corrosion resistance, while reducing weight and eliminating material waste, costly machining, and add-on reinforcing shells.
The features and advantages described in the specification are not all inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter.